The invention relates to an electrophotographic copying apparatus, and more particularly, to an electrophotographic copying machine comprising a photosensitive member for performing electrophotography which member includes a conductive layer carrying a sequential lamination of a first photoconductive layer which is sensitive to visible light and a second photoconductive layer which is sensitive to ultraviolet rays and which serves as an insulator to the visible light and on which member an electrostatic latent image is formed, whereby a plurality of copies are produced by repeatedly using the latent image once formed.
In general, an electrophotographic copying apparatus, as is well recognized, is constructed to produce a copy from different kinds of originals in size and therefore its photosensitive member is formed so as to accommodate the greatest size of originals.
However, when a copy of an original smaller than the greatest size is produced, an area on the photosensitive member where an electrostatic latent image corresponding to an original image to be copied is formed does not cover the entire surface of the photosensitive member. In other words, there exists a part of the entire photosensitive member surace which it does not face a copy sheet to be copied because of a difference in size (hereinafter referred to as an area beyond image scope). When a developing operation is performed on the area beyond image scope on the photosensitive member under its charged condition, a variety of difficulties arises from depositing a developing toner on the area beyond image scope. For example, some of the difficulties which are encountered include a deterioration of a transfer capability of the latent image due to depositing the developing toner on a transfer roller, contamination of copy sheets due to retransferring the developing toner deposited on the transfer roller to the back of the copy sheets, waste of the developing toner, or the like.
The causes that the area beyond image scope on the photosensitive member is charged are as follows.
(a) Charge generated during a charging process for forming an electrostatic latent image is not eliminated.
(b) Charge is directly injected from a transfer roller, to which is normally applied a bias voltage, due to no interposition of a copy sheet during a step for transferring, and the like.
Since these unnecessary charges on a photosensitive member for use in an electrophotographic copying apparatus which utilizes the conventional well-known Carlson's process can be removed only by an irradiation of visible light, there has been disclosed an apparatus that is provided with a lamp for eliminating the charge on the area beyond image scope by an irradiation of visible light prior to a developing operation.
However, even with such charge eliminating means, charge on the area beyond image scope could not be eliminated in an electrophotographic copying process employing a photosensitive member which includes a conductive layer carrying a sequential lamination of a first photoconductive layer which is sensitive to visible light and a second photoconductive layer which is sensitive to ultraviolet rays. This is because once the area beyond image scope has been charged after formation of an electrostatic latent image, charge trapped on the surface of the photosensitive member and on the conductive layer by charge having access to the outside is retrapped to remain on opposite sides of the second photoconductive layer which is not excited even by the irradiation of the visible light alone.
FIG. 1 illustrates a sequence of electrophotographic copying steps for producing a plurality of copy images by repeatedly subjecting an electrostatic latent image formed by the conventional Carlson's process to a developing and a transfer step. As shown, a photosensitive member 1 includes a conductive layer 2 on which a photoconductive layer 3 is laminated. FIGS. 1(I) to (V) respectively show a step for uniform charging of the photosensitive member 1 to the positive polarity by a corona charger 4, a step for an imagewise irradiation 12, a step for developing with a magnet brush 6 having a negatively charged toner, a step for transfer of a toner image on a sheet 7 to be copied with a transfer roller 9 connected to a transfer bias source 8 and a step for a neutralization of charge. Particularly, in the step of FIG. 1(IV), a charge injection onto the area beyond image scope of the photosensitive member 1 will happen as shown. And in the step of FIG. 1(V), in order to eliminate the charge injected onto the area beyond image scope an irradiation of visible light is carried out with a lamp 11 for neutralization which is connected to a power supply 10 only on the area beyond image scope indicated by broken lines with an arrow. Thereafter, by repeatedly performing the steps shown in FIGS. 1(III) to (V) successively, a plurality of copies can be produced with an electrostatic latent image which is once formed on the photosensitive member 1.
Apart from the electrophotographic copying process described above, there has been known such an electrophotographic copying process using a photosensitive member and which includes a conductive layer carrying a sequential lamination of a photoconductive layer and an insulator layer, an electrostatic latent image is formed by trapping charge on opposite sides of the insulator layer during a combination of steps for a corona charging, imagewise exposure, uniform exposure of light rays and the like, and an improvement on such electrophotographic copying process in which a photosensitive layer which is sensitive to ultraviolet rays is utilized on the surface of the photosensitive member instead of the insulator layer. Additionally, the inventor has developed a photosensitive member which includes a conductive layer carrying a sequential lamination of a photoconductive layer which is sensitive to visible light, a filter layer for absorbing ultraviolet rays and a photoconductive layer which is sensitive to ultraviolet rays, and an electrophotographic copying process which utilizes the photosensitive member. In these photosensitive members, the photoconductive layer which is sensitive to ultraviolent rays as well as the conductive layer on the surface thereof serves as an insulator during an irradiation of visible light, not allowing charge to migrate. Therefore, in an electrophotographic copying process employing a photosensitive member which includes such insulator on the surface thereof, even if an attempt is made to remove the charge which is injected into the area beyond image scope during a step for producing a plurality of copies, it will be impossible to eliminate the charge. This will be explained, by way of example, in FIGS. 2(I) through (IV) illustrating an electrophotographic copying process for producing a plurality of copies. As shown in FIG. 2(I), a photosensitive member 21 which includes a conductive layer 22 carrying a sequential lamination of a photoconductive layer 23 which is sensitive to visible light and a light transmitting insulator layer 24 is employed and an electrostatic latent image is formed by trapping charges of opposite polarities on opposite sides of insulator 24. The latent image, which is to be subjected only to a developing and a transfer step to produce a plurality of copies, is developed by a magnet brush 26 which has a negatively charged toner 25 in a developing step shown in FIG. 2(II). In a transfer step shown in FIG. 2(III), the toner image is transferred onto a copy sheet 29 under the action of a transfer roller 28 which is connected to a transfer bias source 27 and to which is applied a bias voltage. Simultaneously with the transfer process a charge is injected from transfer roller 28 onto the area beyond image scope of photosensitive member 21. FIG. 2(IV) illustrates a step in which the useless charge on the area beyond image scope is neutralized by an irradiation of visible light with a neutralization lamp 31 which is connected to a power supply 30 in accordance with the conventional known art. Specifically, the area beyond image scope to be neutralized is shown by broken lines with an arrow. With the area irradiated by visible light, the charge thereon is trapped at the interface between opposite sides of insulator layer 24, failing to be removed. Even with such electrophotographic copying process, a plurality of copies may be produced with an electrostatic latent image which is once formed on photosensitive member 21 by repeatedly performing only steps shown in FIGS. 2(II) through (IV) successively.
Thus, when the photosensitive member is used which includes an insulator layer on the surface thereof, a charge on the area beyond image scope may not be neutralized only by visible light. Consequently, toner is deposited on the area beyond image scope, which has not been neutralized, resulting in disadvantages such as a waste of toner, contamination of a transfer roller and the back of a copy sheet, or the like. It is to be noted that these disadvantages may be caused similarly even when a photoconductive layer which is sensitive to ultraviolet rays but insensitive to visible light is used as a surfacial insulator layer.